The management of the flow of liquids within small diameter channels presents challenges as the scale of the channels and volumes of the liquids are reduced. One significant constraint is the configuration of traditional valve technology. The management of liquid flows in nano-liter volumes encounters significant limitation when the scale of fluid management is severely affected by poorly-swept or dead volume that is inherent within traditional switching methods. The method of using fluid within these nano scale capillaries and channels to act as its own on/off valve by freezing and thawing that liquid is known in the art, see for example U.S. Pat. Nos. 6,159,744 and 5,795,788. It has been found that the flow of liquids can be diverted to a further channel or chamber by merely freezing and thawing the liquid contained within a segment of tubing or channel. This flow-switching device, which is referred to as “freeze-thaw valving”, requires no moving parts within the solvent-wetted path and most importantly contributes no dead volume within the analytical system.
Prior art freeze-thaw valves freeze liquid within a freeze-thaw segment by aspirating a jet of cold gas directly at the freeze-thaw segment. Agents such as liquid carbon dioxide and liquid nitrogen have been used to freeze the contents of the freeze-thaw segment. Unfortunately, the aspiration of the jet of cold gas at the freeze-thaw segment can lead to a build up of frost that acts as insulation, reducing the efficiency of refrigeration and allowing the temperature within the freeze-thaw segment to rise. This elevated temperature, within the freeze-thaw segment, results in the eventual failure of the valve. Additionally, a large volume of gas is needed to operate these prior art freeze-thaw valves.
While prior art freeze-thaw valves utilized a jet of cold gas directed at a freeze-thaw segment, heat pumping based on Peltier principles is a viable method of heat removal to accomplish freezing of the liquid contents of the freeze-thaw segment. Typically, a cascade or series-arrangement of Peltier stages is necessary to attain the required temperatures for rapid freezing of the freeze-thaw segment. Commercially available six-stage cascades are capable of producing temperature differentials as high as 130 degrees Centigrade between the respective hot and cold faces of the heat pump. Unfortunately, such cascades typically do not respond rapidly enough when a drive voltage is applied, because in order to change temperature they must transfer heat associated with their own thermal mass. Additionally, when rapid and/or large-magnitude changes are made in device temperature, thermal stresses induced by such changes contribute to degradation of the device and shortening of device lifespan.
In order to avoid thermal stress problems in Peltier heat pumps used for freeze-thaw valving, it is desirable to operate the heat pump in a mode where it is driven with a substantially constant drive current causing the resulting temperature differential between the hot and cold faces to be substantially constant. Unfortunately, the operation of a Peltier heat pump in a constant pumping mode is not conducive to its use in freeze-thaw valving, because in order to properly utilize freeze-thaw valving it is necessary to be able to both freeze and thaw the same selected segment of a fluidic conduit.